The postpartum period is a time when many women wish to reduce body fat, with exercise often recommended as a means to achieve this. However, concern has been raised that the energy intake of very physically active women may be inadequate for optimal breast milk production. To date, there has been no systematic research on this issue. Both lactation and exercise are known to influence levels of plasma hormones involved in carbohydrate, protein and lipid metabolism. Although increased exercise leads to higher energy requirements. these hormonal changes may promote greater efficiency in breast milk synthesis, thereby, protecting, milk production in the face of high energy expenditure. Our preliminary data comparing exercising women with sedentary women during lactation show no difference in breast milk production, despite a high energy expenditure (3169 kcal/d) in the former group. The objectives of the proposed research are: 1) to evaluate the effect of a 12-week exercise program consisting of aerobic workouts five times per week on breast milk volume and composition, and 2) to determine whether plasma hormone levels (prolactin, insulin, cortisol and T3) are modified by exercise and fluctuations in maternal body composition during the first four months of lactation. The experimental design will be to randomly assign lactating women to one of two groups: a) an exercise group (N=15), in which subjects will begin an exercise program at 7 wks postpartum, or b) a control group (N=15), in which subjects will not undertake vigorous exercise more than once per week during the study. Assessments of maternal dietary intake and activity level (3-day record), body composition (hydrostatic weighing and skinfolds), resting metabolic rate (indirect calorimetry), plasma hormone levels, breast milk volume (3-day test-weighing), milk composition (24-hr sampling) and infant growth will be made at baseline (6-7 wks) and at 12-13 and 19-20 wks postpartum. Maximum oxygen uptake (V02 max) will be measured at 6-7 and 19-20 wks as an indicator of aerobic fitness. The results of this study will be important not only from the standpoint of public health policy but also in understanding the metabolic interrelationships between maternal nutritional status, physical activity, and lactation performance.